Polymer composites with high dielectric constant are important functional materials with important application value in many fields such as the insulation, electromechanical and biological engineering, etc. Introducing electric conductive into polymers is one main route for preparing polymer composites with high dielectric constant. Owing to outstanding electrical properties, large length to diameter ratio and good mechanical properties, carbon nanotubes have attracted much attention. To date, a variety of carbon nanotube/polymer composites have been prepared. As a conductor/polymer composite, the carbon nanotube/polymer composite undergoes an insulator-conductor transition when the content of carbon nanotubes approaches the percolation threshold, resulting in a high dielectric constant, but also tends to have high dielectric loss. On the other hand, low percolation thresholds are undoutedly attractive in order to maintain good processability of the polymer and to reduce processing costs.
At present, forming a coating on the surface of the electric conductor is an effective method to reduce dielectric loss. The coating can prevent the mutual contact of conductors, thereby depressing the leakage current in composites. However, the corresponding composites based on the coated fillers usually enlarge percolation thresholds (fc values). To overcome this problem, people try to form a double percolation structure, to achieve the goal of reducing the percolation threshold. In detail, nanofillers are selectively distributed in one phase of the immiscible polymer blend, or selective distribution of nanofillers at the interface to form the percolated network structure, and thus reducing the content of nanofillers. For example, before this invention, Dang's group introduced multi-walled carbon nanotubes (MWCNTs) into polystyrene (PS)/poly(vinylidene fluoride) (PVDF) blend that has double continuous phase morphology, and found that with the same content of MWCNTs, the dispersion of MWCNTs in different phase played a significant role on dielectric properties of the composites. However, the maximum dielectric constant that MWCNT/PS/PVDF composites exhibited was 485 at 100 Hz, and at this condition, the loading of MWCNTs was as high as 3.9 vol % (please see: Xiaodong Zhao, Jun Zhao, Jianping Cao, Dongrui Wang, Guohua Hu, Fenghua Chen, Zhimin Dang. Effect of the selective localization of carbon nanotubes in polystyrene/poly(vinylidene fluoride) blends on their dielectric, thermal, and mechanical properties. Materials and Design. 2014, 56: 807-815). This group also made effort on dispersing MWCNTs at the interface between PS and PVDF, and found that when f=0.4-0.6 wt %, the composite has the maximum dielectric constant at 100 Hz, which is 398; while the dielectric loss is also very high, which is 0.8-200 at 100 Hz. These interesting researches have demonstrated that using a polymer blend with double continuous morphology is beneficial to prepare Hik-PNCs with lower fc, but this kind of polymeric matrix seems not effective for simultaneously obtaining high dielectric constant and low dielectric loss. What's more, complicate structural design seems not effective for obviously improve the integrated dielectric performances, and introduction of more polymers makes the processing become difficult to be controlled.
Based on the overall background described above, it is still a great challenge to explore a new and effective method of preparing electric conductor/polymer composites with high dielectric constant, low dielectric loss and low fc.